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Clinical Sciences |

Prediction of Retinopathy of Prematurity Using the Screening Algorithm WINROP in a Mexican Population of Preterm Infants FREE

Luz Consuelo Zepeda-Romero, MD, MSc; Anna-Lena Hård, MD, PhD; Larissa Maria Gomez-Ruiz, MD; Jose Alfonso Gutierrez-Padilla, MD, MSc; Eusebio Angulo-Castellanos, MD; Juan Carlos Barrera-de-Leon, MD, PhD; Juan Manuel Ramirez-Valdivia, MD; Cesareo Gonzalez-Bernal, MD; Claudia Ivette Valtierra-Santiago, MD; Esperanza Garnica-Garcia, MD; Chatarina Löfqvist, PhD; Ann Hellström, MD, PhD
[+] Author Affiliations

Author Affiliations: Retinopathy of Prematurity Clinic and Blindness Prevention (Drs Zepeda-Romero, Valtierra-Santiago, and Garnica-Garcia), Departments of Neonatology, Unidad J. I. Menchaca (Drs Gomez-Ruiz and Ramirez-Valdivia) and Unidad F. Antonio Alcalde (Drs Gutierrez-Padilla and Angulo-Castellanos), Hospital Civil de Guadalajara, Centro Universitario de Ciencias de la Salud, Universidad de Guadalajara, and Department of Neonatology, Hospital Materno Infantil Esperanza Lopez Mateos (Drs Barrera-de-Leon and Gonzalez-Bernal), Guadalajara, Jalisco, Mexico; and Department of Ophthalmology, Institute of Neuroscience and Physiology, University of Gothenburg, Gothenburg, Sweden (Drs Hård, Löfqvist, and Hellström).


Arch Ophthalmol. 2012;130(6):720-723. doi:10.1001/archophthalmol.2012.215.
Text Size: A A A
Published online

Objective To retrospectively validate the WINROP (weight, insulin-like growth factor I, neonatal, retinopathy of prematurity [ROP]) algorithm in identification of type 1 ROP in a Mexican population of preterm infants.

Methods In infants admitted to the neonatal intensive care unit at Hospital Civil de Guadalajara from 2005 to 2010, weight measurements had been recorded once weekly for 192 very preterm infants (gestational age [GA] <32 weeks) and for 160 moderately preterm infants (GA ≥32 weeks). Repeated eye examinations had been performed and maximal ROP stage had been recorded. Data are part of a case-control database for severe ROP risk factors.

Results Type 1 ROP was found in 51.0% of very preterm and 35.6% of moderately preterm infants. The WINROP algorithm correctly identified type 1 ROP in 84.7% of very preterm infants but in only 5.3% of moderately preterm infants. For infants with GA less than 32 weeks, the specificity was 26.6%, and for those with GA 32 weeks or more, it was 88.3%.

Conclusions In this Mexican population of preterm infants, WINROP detected type 1 ROP early in 84.7% of very preterm infants and correctly identified 26.6% of infants who did not develop type 1 ROP. Uncertainties in dating of pregnancies and differences in postnatal conditions may be factors explaining the different outcomes of WINROP in this population.

Retinopathy of prematurity (ROP) is a major cause of life-long blindness; its frequency is strongly associated with the quality of health care. In high-income countries, where many very immature babies survive, severe ROP affects the most immature, and screening and treatment programs make blindness rare. In middle-income countries, health care is good enough for survival of some extremely premature and more mature babies1 but is insufficient in preventing ROP, leading to an increased prevalence of ROP in more mature babies; an epidemic of ROP-related blindness is presently seen in these countries. In the poorest parts of the world, where immature babies do not survive, ROP is not a problem.2

Current screening programs are based on gestational age (GA) and/or birth weight (BW) but, because of national differences in socioeconomic status and quality of care, different countries need different screening criteria.2 The disadvantage of using GA for national screening criteria was recently shown in a study3 from Rio de Janeiro. Two clinics with high survival rates had no infants with type 1 ROP whose GA was more than 32 weeks, while in 5 other clinics with poorer survival rates, infants with GA 35 weeks or less required screening.

In high-income countries that screen infants with GA less than 32 weeks, only 5% to 10% of the infants need treatment,4 and many fragile babies who will never develop sight-threatening ROP undergo repeated painful and stressful eye examinations.5

Based on the finding of the association between poor early weight gain,6 low serum insulin-like growth factor I, and ROP and in an attempt to refine ROP screening, the algorithm WINROP (weight, insulinlike growth factor I, neonatal, ROP) was developed and validation of its ability to predict severe ROP was performed.7,8 Later, WINROP was found to function well using only weights,9 allowing blood sampling and analyses to be omitted. Studies validating WINROP in 3 different populations with GA less than 32 weeks have been published. In one Swedish9 and one US population,10 sensitivity of 100% and specificity of 84.5% and 81.7%, respectively, were found, and in a Brazilian study,11 sensitivity was 90.5% and specificity was 55.0%.

The aim of this study was to validate WINROP regarding its ability to predict severe ROP, defined as type 1 ROP, in a Mexican population of preterm infants.

PATIENTS

The study group comprised a case-control database of infants (N = 377) who were screened and/or treated for ROP at the Hospital Civil de Guadalajara, Mexico, from April 20, 2005, to April 15, 2010, and had weekly weight measurements. The infants included had survived from the initial ophthalmologic examination at 3 to 6 weeks to the final ROP examination. Fifteen infants were referred for treatment from Hospital Materno Infantil Esperanza Lopez Mateos in Guadalajara.

Hospital Civil de Guadalajara is a university hospital located in Guadalajara, a city with a total population of approximately 5 million inhabitants. Guadalajara is the main city in the state of Jalisco. All admitted patients were from the low-income urban population in the Guadalajara region. The neonatal intensive care unit ROP program screens approximately 220 preterm babies per year. Each weekly weight was entered into WINROP.

The neonatologists determined the GA for each infant on the basis of the mother's report of the date of her last menstrual period.

Ophthalmologic examinations consisted of binocular indirect ophthalmoscopy after pupillary dilatation with tropicamide, 0.5%, and phenylephrine, 2.5%, using a +28-diopter lens and a lid speculum; when necessary, a depressor (Flynn Scleral) was used to better identify peripheral retinal alterations. Retinopathy was classified according to the International Classification of Retinopathy of Prematurity (stages 1-5).12 The indications for treatment were type 1 ROP according to the recommendations of the ETROP (Early Treatment for Retinopathy of Prematurity) study.13 The same ROP-experienced pediatric ophthalmologist (L.C.Z.R.) performed all ophthalmic examinations.

WINROP SCREENING

The first step of the WINROP algorithm was developed using the methods of online statistical surveillance.1416 Briefly, the algorithm, using data on preterm infants with no ROP or with ROP stage 1, calculates the expected “safe” weight for each child. The difference between the expected weight gain and documented values of postnatal weight development at each time point is calculated and accumulated. If the accumulated sum reaches a threshold limit, an alarm is triggered.7 Data were retrieved from the files regarding weight once every postnatal week until 35 to 36 postmenstrual weeks; a person unaware of the maximum ROP stage entered these data, including GA at birth and BW, into the surveillance algorithm. Weight was measured every morning before feeding, and the weight on every 7th day was recorded in WINROP. In the first step of the algorithm, an alarm statistic is calculated for each child to judge whether there is enough evidence to conclude that a significant slowing of growth has occurred.

In previous studies, WINROP evaluations were at 1 of 3 levels: (1) no alarm, (2) alarm at low risk, and (3) alarm at high risk.7,8 Risk level was based on GA and BW. Because classifying alarms as either high risk or low risk before 33 weeks' postmenstrual age (PMA) or no alarm or low risk after 33 weeks' PMA has been used successfully in previous studies, we decided to simplify classification into alarms before 33 weeks' PMA or no alarms before 33 weeks' PMA. In case of an alarm, the PMA (week) was registered as displayed by the system. After the WINROP analysis, the results were correlated with the maximum ROP stage for each child as extracted from the child's files.

STATISTICAL ANALYSIS

The surveillance system using the variable weight gain was evaluated regarding sensitivity (probability that an alarm is triggered given that the infant develops type 1 ROP) and specificity (probability that an alarm is not triggered given that the infant will not develop type 1 ROP). The negative and positive predictive values were calculated using the sensitivity, specificity, and prevalence in the present study group. We calculated 95% CIs for estimated binary proportions (sensitivity and specificity) using the exact method of Clopper-Pearson.17

ETHICS

The Research Ethics Committee of the Hospital Civil de Guadalajara approved the study protocol regarding postnatal weight gain and ROP. The protocol also conforms to the provisions of the Declaration of Helsinki in 1995 (as revised in Edinburgh, 2000).

ROP OUTCOME

A total of 362 infants had been screened for ROP and had weekly weight measurements performed at Hospital Civil de Guadalajara and 15 at Hospital Materno Infantil Esperanza Lopez Mateos in Guadalajara. Because WINROP was developed for infants with GA less than 32 weeks, separate analyses were performed for infants with GA less than 32 weeks (very preterm) and for those with GA 32 weeks or more (moderately preterm).

Infants With GA Less Than 32 Weeks

There were 206 very preterm infants, 14 of whom were excluded (12 owing to hydrocephalus, which may cause nonphysiologic weight gain, and 2 because of unspecified ROP stage), leaving a study group of 192 infants (median GA, 30 weeks; range, 25-31 weeks). The type of ROP in relation to WINROP alarm in very preterm infants is given in Table 1. In this group, 163 infants had ROP (84.9%) and more than half (51.0%) had type 1 ROP. The ROP stages of these infants are also given in Table 1. Of the 192 infants, 102 (53.1%) were diagnosed as having plus disease.

Table Graphic Jump LocationTable 1. Type of ROP in Relationship With WINROP Alarm in Infants With GA Less Than 32 Weeks
Infants With GA 32 Weeks or More

There were 171 moderately preterm infants. Two of them were excluded because of unspecified stage of ROP, 4 because of hydrocephalus, and 5 for an insufficient number of weight measurements. The moderately preterm study group thus comprised 160 infants (median GA, 33 weeks; range, 32-35 weeks). The type of ROP in relation to WINROP alarm in moderately preterm infants is provided in Table 2. In this group, 103 infants (64.4%) had ROP and more than one-third had type 1 ROP. The ROP stages of these infants are also provided in Table 2. Of the 160 moderately preterm infants, 63 (39.4%) had plus disease.

Table Graphic Jump LocationTable 2. Type of ROP in Relationship With WINROP Alarm in Infants With GA 32 Weeks or Morea
WINROP OUTCOME

For all 352 infants, weekly weights had been registered and were entered into the online surveillance system. Details follow on the outcome of using the WINROP screening tool.

Infants With GA Less Than 32 Weeks

For 40 children (20.8%), no alarm was triggered (Table 1). Fifteen of these infants developed type 1 ROP. The median GA of these 15 patients was 31 weeks (range, 28-31 weeks), and the median BW was 1420 g (range, 1180-1510 g). Six of the 15 infants with type 1 ROP with no alarms had stage 2+ ROP in zone II. For 152 infants (79.2%), an alarm was triggered. In this group, 83 (54.6%) developed type 1 ROP. The sensitivity with which WINROP predicted type 1 ROP in the infants with GA less than 32 weeks was 84.7%, and the specificity was 26.6%.

Infants With GA 32 Weeks or More

No alarm was generated for 145 of the infants (90.6%) in this group, although 54 of them had type 1 ROP. Of the 5 infants with type 2 ROP without alarms, none developed type 1 ROP. Fifteen infants had alarms, and 3 of them developed type 1 ROP. The sensitivity of WINROP in predicting type 1 ROP in infants with GA 32 weeks or more was 5.3%, and the specificity was 88.3%.

In this retrospective study on a Mexican population of preterm infants, the WINROP algorithm correctly identified type 1 ROP in 83 of 98 very preterm infants (84.7%) (GA <32 weeks) but only in 3 of 57 of moderately preterm infants (5.3%) (GA ≥32 weeks). In previous studies from Sweden9 and North America,10 100% of infants developing stage 3 ROP have been identified. There may be several reasons for this discrepancy.

In the previous studies,9,10 GA calculations were based on ultrasound measurements during early pregnancy; in this study, the mother's report on her last menstrual period was the basis of GA estimation, making it less accurate.

In previous studies, we have tested WINROP's ability to predict proliferative ROP, and we decided not to include plus disease because it is difficult to evaluate retrospectively. For the population of the present study, data on plus disease were provided, and because future prospective studies will aim to predict ROP that needs to be treated according to the criteria recommended by the ETROP study,13 we set out to determine the ability of WINROP to predict type 1 ROP in this population. Thus, some infants with proliferative ROP without plus disease or in zone III are not included among the patients we wanted to identify.

Before the study, we had hypothesized that all infants with an alarm before 33 weeks' PMA, ie, before the age when proliferative ROP is likely to occur, would need eye examinations, while those without such alarms would not.

According to present treatment criteria, patients should be treated if they have stage 2+ ROP in zone II that is not proliferative. A total of 31 infants (10 with GA <32 weeks) in this study had stage 2+ ROP in zone II. In the Swedish experience, stage 2+ ROP in zone II is very rare and was not found in any patient in the Gothenburg study.9 In the ETROP study,13 only 66 of 828 infants (8.0%) with BW less than 1251 g and prethreshold disease had stage 2+ ROP in zone II compared with 31 of 352 (8.8%) of all screened infants in the present study. A high frequency of stage 2+ ROP in zone II has also been found in India, according to A. Vinekar, MD, PhD (e-mail communication, April 2011).

The fact that a different kind of ROP affecting more mature babies is seen in developing countries has led to different screening criteria in terms of GA and BW in different populations (≤34 weeks weight for GA and ≤1750 g BW in Mexico).18 The finding of severe ROP in comparatively mature infants and the finding of more stage 2+ in zone II in this Mexican population and an Indian population may indicate different quality of neonatal care, with risk factors and causative mechanisms that result in different progression of ROP in different populations. In the present study, we know that oxygen monitoring was insufficient and that the antenatal care could be improved,1921 and there is reason to believe that other differences exist between this population and those of high-income countries that could be improved by better care. Because WINROP gives very early indications of poor growth that is related to later morbidity, one may anticipate that it can be used as an indicator of the need for better early care long before information on ROP outcome is available, possibly at a stage when morbidity may be prevented.

Considering the possible differences in ROP progression in this population compared with those of high-income countries, one cannot be confident that treatment criteria necessarily should be the same as those based on US infants with BW less than 1251 g, as in the ETROP study. One intriguing factor for this and other populations is the importance of plus disease as a criterion for type 1 ROP and the inconsistencies found between observers regarding plus disease, making overdiagnosis a possible problem.22

In the present study of a Mexican population, teenage pregnancies were common, GA estimates were less accurate than in other WINROP studies, and prenatal and postnatal care were different.

It is not surprising that WINROP, which was developed for use in infants with GA less than 32 weeks and initiates an alarm before 33 weeks' PMA, was not able to predict type 1 ROP in infants with GA 32 weeks or more. Of those with GA less than 32 weeks, 84.7% received an alarm. We do not know to what extent inaccurate dating contributed to WINROP's poorer ability to predict ROP in very preterm infants in this study compared with previous studies. The exact date of birth is necessary, and one must know whether the baby is born before 32 weeks' GA. Of the 15 infants with type 1 ROP and no alarms, 12 had GA 30 weeks or more, making inappropriate dating of 2 weeks significant. The results from the moderately preterm infants show that WINROP is not useful for infants for whom it was not developed.

Early weight gain is a marker for conditions that later may lead to ROP and possibly other morbidities. Because WINROP generates early alarms regarding later morbidity, its use as a monitor of quality of early care of infants with GA less than 32 weeks should be explored. Early alarms may lead to interventions preventing disease. Discrepancies could reflect the excess of ROP in these settings that is a result of differences in the quality of neonatal care.

Correspondence: Ann Hellström, MD, PhD, Section of Pediatric Ophthalmology, The Queen Silvia Children's Hospital, The Sahlgrenska Academy at University of Gothenburg, S-416 85 Göteborg, Sweden (ann.hellstrom@medfak.gu.se).

Submitted for Publication: September 11, 2011; final revision received January 2, 2012; accepted January 5, 2012.

Financial Disclosure: PremaCure AB has rights to WINROP, and Drs Hård, Löfqvist, and Hellström own shares in a company controlling PremaCure AB.

Additional Contributions: Jill Walker provided data handling and Niclas Nilsson at Activa provided the beta version of WINROP. Ulf Nyman, MD, PhD, Trelleborg Hospital, Sweden, gave valuable statistical advice.

Profit J, Lee D, Zupancic JA,  et al.  Clinical benefits, costs, and cost-effectiveness of neonatal intensive care in Mexico.  PLoS Med. 2010;7(12):e1000379Link to Article
Link to Article
Gilbert C. Retinopathy of prematurity: a global perspective of the epidemics, population of babies at risk and implications for control.  Early Hum Dev. 2008;84(2):77-82
PubMed   |  Link to Article
Zin AA, Moreira MEL, Bunce C, Darlow BA, Gilbert CE. Retinopathy of prematurity in 7 neonatal units in Rio de Janeiro: screening criteria and workload implications.  Pediatrics. 2010;126(2):e410-e417
PubMed  |  Link to Article   |  Link to Article
Quinn GE, Gilbert C, Darlow BA, Zin A. Retinopathy of prematurity: an epidemic in the making.  Chin Med J (Engl). 2010;123(20):2929-2937
PubMed
Rush R, Rush S, Ighani F, Anderson B, Irwin M, Naqvi M. The effects of comfort care on the pain response in preterm infants undergoing screening for retinopathy of prematurity.  Retina. 2005;25(1):59-62
PubMed   |  Link to Article
Wallace DK, Kylstra JA, Phillips SJ, Hall JG. Poor postnatal weight gain: a risk factor for severe retinopathy of prematurity.  J AAPOS. 2000;4(6):343-347
Link to Article
Löfqvist C, Andersson E, Sigurdsson J,  et al.  Longitudinal postnatal weight and insulin-like growth factor I measurements in the prediction of retinopathy of prematurity.  Arch Ophthalmol. 2006;124(12):1711-1718
PubMed   |  Link to Article
Löfqvist C, Hansen-Pupp I, Andersson E,  et al.  Validation of a new retinopathy of prematurity screening method monitoring longitudinal postnatal weight and insulinlike growth factor I.  Arch Ophthalmol. 2009;127(5):622-627
PubMed   |  Link to Article
Hellström A, Hård AL, Engström E,  et al.  Early weight gain predicts retinopathy in preterm infants: new, simple, efficient approach to screening.  Pediatrics. 2009;123(4):e638-e645
PubMed  |  Link to Article   |  Link to Article
Wu C, Vanderveen DK, Hellström A, Löfqvist C, Smith LE. Longitudinal postnatal weight measurements for the prediction of retinopathy of prematurity.  Arch Ophthalmol. 2010;128(4):443-447
PubMed   |  Link to Article
Hård AL, Löfqvist C, Fortes Filho JB, Procianoy RS, Smith L, Hellström A. Predicting proliferative retinopathy in a Brazilian population of preterm infants with the screening algorithm WINROP.  Arch Ophthalmol. 2010;128(11):1432-1436
PubMed   |  Link to Article
Committee for the Classification of Retinopathy of Prematurity.  An international classification of retinopathy of prematurity.  Arch Ophthalmol. 1984;102(8):1130-1134
PubMed   |  Link to Article
Early Treatment for Retinopathy of Prematurity Cooperative Group.  Revised indications for the treatment of retinopathy of prematurity: results of the Early Treatment for Retinopathy of Prematurity randomized trial.  Arch Ophthalmol. 2003;121(12):1684-1694
PubMed   |  Link to Article
Frisén M. Statistical surveillance: optimality and methods.  Int Stat Rev. 2003;71(2):403-434
Link to Article
Shiryaev AN. On optimum methods in quickest detection problems.  Theory Probab Appl. 1963;8(1):22-46Link to Article
Link to Article
Roberts SW. A comparison of some control chart procedures.  Technometrics. 1966;8(3):411-430
Hollander M, Wolfe DA. Nonparametric Statistical Methods. 2nd ed. New York, NY: John Wiley & Sons; 1999
Secretaria de Salud Mexico.  Lineamiento para el tratamiento de la retinopatia del recien nacido prematuro 2007. http://www.v2020la.org/docs/lineamiento/Lineamiento-ROP-SSA-2007.pdf. Accessed March 21, 2012
Frank R, Pelcastre B, Salgado de Snyder VN, Frisbie WP, Potter JE, Bronfman-Pertzovsky MN. Low birth weight in Mexico: new evidence from a multi-site postpartum hospital survey.  Salud Publica Mex. 2004;46(1):23-31
PubMed   |  Link to Article
Pérez Molina JJ, Cobián López BE, Silva Maciel CA. Maternal risk factors and premature birth in a public hospital at west of Mexico.  Ginecol Obstet Mex. 2004;72:142-149
PubMed
Perez-Molina JJ, Blancas Jacobo O, Ramírez Valdivia JM. Enfermedad de membrana hialina: mortalidad y factores de riesgo maternos y neonatales.  Ginecol Obstet Mex. 2006;74(7):353-359
PubMed
Chiang MF, Jiang L, Gelman R, Du YE, Flynn JT. Interexpert agreement of plus disease diagnosis in retinopathy of prematurity.  Arch Ophthalmol. 2007;125(7):875-880
PubMed   |  Link to Article

Figures

Tables

Table Graphic Jump LocationTable 1. Type of ROP in Relationship With WINROP Alarm in Infants With GA Less Than 32 Weeks
Table Graphic Jump LocationTable 2. Type of ROP in Relationship With WINROP Alarm in Infants With GA 32 Weeks or Morea

References

Profit J, Lee D, Zupancic JA,  et al.  Clinical benefits, costs, and cost-effectiveness of neonatal intensive care in Mexico.  PLoS Med. 2010;7(12):e1000379Link to Article
Link to Article
Gilbert C. Retinopathy of prematurity: a global perspective of the epidemics, population of babies at risk and implications for control.  Early Hum Dev. 2008;84(2):77-82
PubMed   |  Link to Article
Zin AA, Moreira MEL, Bunce C, Darlow BA, Gilbert CE. Retinopathy of prematurity in 7 neonatal units in Rio de Janeiro: screening criteria and workload implications.  Pediatrics. 2010;126(2):e410-e417
PubMed  |  Link to Article   |  Link to Article
Quinn GE, Gilbert C, Darlow BA, Zin A. Retinopathy of prematurity: an epidemic in the making.  Chin Med J (Engl). 2010;123(20):2929-2937
PubMed
Rush R, Rush S, Ighani F, Anderson B, Irwin M, Naqvi M. The effects of comfort care on the pain response in preterm infants undergoing screening for retinopathy of prematurity.  Retina. 2005;25(1):59-62
PubMed   |  Link to Article
Wallace DK, Kylstra JA, Phillips SJ, Hall JG. Poor postnatal weight gain: a risk factor for severe retinopathy of prematurity.  J AAPOS. 2000;4(6):343-347
Link to Article
Löfqvist C, Andersson E, Sigurdsson J,  et al.  Longitudinal postnatal weight and insulin-like growth factor I measurements in the prediction of retinopathy of prematurity.  Arch Ophthalmol. 2006;124(12):1711-1718
PubMed   |  Link to Article
Löfqvist C, Hansen-Pupp I, Andersson E,  et al.  Validation of a new retinopathy of prematurity screening method monitoring longitudinal postnatal weight and insulinlike growth factor I.  Arch Ophthalmol. 2009;127(5):622-627
PubMed   |  Link to Article
Hellström A, Hård AL, Engström E,  et al.  Early weight gain predicts retinopathy in preterm infants: new, simple, efficient approach to screening.  Pediatrics. 2009;123(4):e638-e645
PubMed  |  Link to Article   |  Link to Article
Wu C, Vanderveen DK, Hellström A, Löfqvist C, Smith LE. Longitudinal postnatal weight measurements for the prediction of retinopathy of prematurity.  Arch Ophthalmol. 2010;128(4):443-447
PubMed   |  Link to Article
Hård AL, Löfqvist C, Fortes Filho JB, Procianoy RS, Smith L, Hellström A. Predicting proliferative retinopathy in a Brazilian population of preterm infants with the screening algorithm WINROP.  Arch Ophthalmol. 2010;128(11):1432-1436
PubMed   |  Link to Article
Committee for the Classification of Retinopathy of Prematurity.  An international classification of retinopathy of prematurity.  Arch Ophthalmol. 1984;102(8):1130-1134
PubMed   |  Link to Article
Early Treatment for Retinopathy of Prematurity Cooperative Group.  Revised indications for the treatment of retinopathy of prematurity: results of the Early Treatment for Retinopathy of Prematurity randomized trial.  Arch Ophthalmol. 2003;121(12):1684-1694
PubMed   |  Link to Article
Frisén M. Statistical surveillance: optimality and methods.  Int Stat Rev. 2003;71(2):403-434
Link to Article
Shiryaev AN. On optimum methods in quickest detection problems.  Theory Probab Appl. 1963;8(1):22-46Link to Article
Link to Article
Roberts SW. A comparison of some control chart procedures.  Technometrics. 1966;8(3):411-430
Hollander M, Wolfe DA. Nonparametric Statistical Methods. 2nd ed. New York, NY: John Wiley & Sons; 1999
Secretaria de Salud Mexico.  Lineamiento para el tratamiento de la retinopatia del recien nacido prematuro 2007. http://www.v2020la.org/docs/lineamiento/Lineamiento-ROP-SSA-2007.pdf. Accessed March 21, 2012
Frank R, Pelcastre B, Salgado de Snyder VN, Frisbie WP, Potter JE, Bronfman-Pertzovsky MN. Low birth weight in Mexico: new evidence from a multi-site postpartum hospital survey.  Salud Publica Mex. 2004;46(1):23-31
PubMed   |  Link to Article
Pérez Molina JJ, Cobián López BE, Silva Maciel CA. Maternal risk factors and premature birth in a public hospital at west of Mexico.  Ginecol Obstet Mex. 2004;72:142-149
PubMed
Perez-Molina JJ, Blancas Jacobo O, Ramírez Valdivia JM. Enfermedad de membrana hialina: mortalidad y factores de riesgo maternos y neonatales.  Ginecol Obstet Mex. 2006;74(7):353-359
PubMed
Chiang MF, Jiang L, Gelman R, Du YE, Flynn JT. Interexpert agreement of plus disease diagnosis in retinopathy of prematurity.  Arch Ophthalmol. 2007;125(7):875-880
PubMed   |  Link to Article

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